Polymeric aromatic carbamic acid esters (polyurethanes) such as diphenylmethane dicarbamates and the related higher homologs, polymethylene polyphenyl carbamates, have become increasingly important products particularly, for use in the preparation of the commercially valuable diphenylmethane diisocyanates and mixtures of diisocyanates and the polyisocyanates by the decomposition of such polymeric aromatic carbamic acid esters in a suitable solvent as shown in Rosenthal et al, U.S. Pat. Nos. 3,962,302 and 3,919,279.
Diphenylmethane diisocyanates and polyisocyanates, available commercially, have been largely produced by the phosgenation of mixtures of diamines and polyamines obtained by the condensation of aniline and formaldehyde with catalytic quantities of a mineral acid as disclosed in Pistor et al, U.S. Pat. No. 4,014,914. German Pat. No. 2,425,658 describes a process wherein the polyisocyanate product of a phosgenation reaction was distilled under reduced pressure. An overhead temperature of 200.degree.-30.degree. C. at 2-2.5 mm. Hg. were the distillation conditions. The distillate mixtures contained 5 to 70% diphenylmethane diisocyanate (both 2,4' and 4,4' isomers). Cooling the distillate caused crystallization of both isomers of the product diphenylmethane diisocyanate.
A proposed prior art process for the preparation of polymeric aromatic carbamic acid esters (polyurethanes) is disclosed in Klauke et al U.S. Pat. No. 2,946,768 and involves the condensation of aryl carbamic acid esters with carbonyl compounds in a dilute aqueous acid condensation medium. However, in such process the carbonyl compound such as formaldehyde tends to react at the nitrogen of the carbamate to produce, along with desired polyurethanes, varying amounts, i.e., generally between 15 percent and 50 percent by weight, of undesirable (alkoxycarbonyl)-phenylaminomethylphenyl compounds which includes the various dimers, trimers, tetramers, etc. of such compounds (also referred to herein as "N-benzyl" compounds). Attempts to prepare mono or diisocyanates and polyisocyanates or to otherwise use the mixture containing the undesired N-benzyl compounds (which cannot be converted to an isocyanate by pyrolysis) and polyurethanes presents many problems since there is no known method for separating the polyurethanes from the N-benzyl impurities.
However, the undesired N-benzyl compounds may be catalytically rearranged to a desired polyurethane in accordance with the teachings of Shawl et al, U.S. Pat. No. 4,146,727. Accordingly, a product mixture from a condensation as disclosed in aforementioned U.S. Pat. No. 2,946,768 containing diurethanes and polyurethanes, N-benzyl compounds, unreacted alkyl phenylcarbamates and other by-products such as amines may be contacted at temperatures of from about 50.degree. C. to 170.degree. C. with a protonic acid medium having a strength at least equal to a 75 percent sulfuric acid such as concentrated sulfuric acid or an acid medium comprising a Lewis acid having a concentration of at least 0.5 percent by weight based on the total reaction mixture, while maintaining a minimum amount of water in the system, to catalytically convert or rearrange said N-benzyl compounds.
Even after rearrangement of the N-benzyl compounds the reaction mixture contains an undesirable amount of detrimental by-products, unreacted starting material and solvent. In order to obtain good yields of polyisocyanate with the maximum content of isocyanate moieties, it would be ideal to enrich or maximize the polyurethane content of the reaction mixture relative to the other components.